This invention relates to a gate valve of the type in which the valve gate is moved between open and closed positions by a rising stem, that is, a stem extending from a bonnet in the top of the valve, that is, by a mechanism externally of the valve moved up and down to move the gate between open and closed positions. These types of valves are popular in industrial applications since they make possible a valve structure having a full opening, that is, when the valve is open the passageway is completely unobstructed. Further, this type of valve has the capability of securely closing fluid flow that is most important when the valve is used in high pressure applications.
Since this type of valve is particularly adaptable in high pressure applications and since it involves a stem that travels the full length of the movement required to fully open and fully close the gate within the valve, the packing around the stem that prevents fluid from flowing from the interior of the valve body to the environment must be replaced more frequently than with other types of valves which employ only a rotating stem. A typical procedure in industry is to provide packing around the valve stem including means to compress the packing so as to prevent leakage and the usual means includes provision for adjusting the compression on the packing so as the packing wears the compression can be increased to shut off leakage of fluid past the valve stem and therefore extend the useful life of the valve packing. However, eventually the packing requires replacement and in many valve applications in use today, replacing the packing around a stem of a rising stem valve requires that all substantial pressure within the valve body be removed, that is, many valves in use today have no provision for retaining pressure within the valve body when the stem packing is removed. This sometimes means shutting down significant processing operations.
To prevent having to shut down a process in order to remove pressure from a rising stem valve, others have suggested a back seat arrangement on the valve that functions to close the valve stem when the valve gate is in a fully opened position, that is, when the valve stem is in the fully raised position so as to permit removing the stem packing. This system is accomplished by providing a back seat on the valve stem, that is, an enlarged diameter portion on the valve stem that provides an annular seating surface thereon which can, when the valve is in the fully opened position, engage a sealing surface in a portion of the valve that guides the valve stem so as to permit removal of valve stem packing. Such arrangement has also been employed in valves to add fire protection, that is, since the back seat feature on a valve stem can be designed in a way so that sealing is accomplished by metal-to-metal seals, rather than using elastomeric seals that would fail more readily in the event of fire.
For background information relating to the subject of this invention, please see the following United States Patents:
______________________________________ PATENT NO. INVENTOR TITLE ______________________________________ 2305590 Marburg et al Pressure Sealed Joint 2426392 Fennema Pressure Sealed Joint 2787439 Bredtschneider Valve Construction 3152786 Soderberg et al Bifaced Gate Valve Having Tapered Seat Sealing Surfaces 3162453 Magos et al Combined Valve Stuffing Box and Bonnet Mounting 3993285 Conley Double Disc Gate Valve With Entrapped Stem Connection 4272055 Herd Single Double Backseat 4289317 Kuc Pump Shaft Closure 4294427 Cilny Mono-Seat Gate Valve 4356832 Velan High Pressure Bonnetless Valve 4601304 Schobl Valve Assembly 4878677 Larkins et al Shut Off Seal About A Shaft Of A Device Having A Side Entry Into A Tank 5190264 Boger Adjustable Valve Packing Arrangement 5234194 Smith Seal For A Shaft 5238252 Stewen et al Shut-Off Fittings With A Sealing Device 5288055 Fosnight et al Brace For A Valve Packing Gland Flange 5435520 Vyvial Backseat Assembly For An Expanding Gate Valve 5454547 Brown Sleeved Seal For A Valve ______________________________________